1. Field of the Invention
The present invention relates to a switch, and more particularly to a magnetically operated proximity switch usable in, for example, a security system.
2. Discussion of the Related Art
While magnetic proximity switches are often confused with magnetic sensors, magnetic proximity switches represent a category of technology exclusive of magnetic sensors. Conventional magnetic proximity switches comprise some combination of magnetic material, electrical contacts, and a mechanism to effect switching. At least one of the electrical contacts is attached to a coiled spring or leaf spring. A reed switch is one example of a conventional magnetic proximity switch.
There are three basic types of reed switches: the dry reed, the mercury wetted reed, and the mercury wetted contact switching capsule. The magnetic behavior of all three reed switch types are similar, but the switches of the mercury type are restricted to certain mechanical orientations because they rely on gravity for successful operation. Mercury based switches also represent an environmental disposal hazard and are more expensive than the dry reed type.
FIG. 13 shows a perspective view of a conventional reed switch 100. The reed switch includes an electrically connecting lead wire 102 to which a flexible magnetically active electric contact member 104 is fixed, an electrically conducting lead wire 106 to which a nonmagnetically active rigid electrical contact member 108 is fixed, an optional lead wire 110 to which a magnetically active rigid electrical contact member 112 may be fixed. The system is enclosed in a hermetic glass envelope 114. In the absence of a magnetic field, the electrical contact 104 is in electrical contact with the non-magnetic electrical contact 108 as a result of a mechanical spring action created by the contact member 104. The lead and the electrical contacts 102, 104, 110, and 112 are composed of high permeability magnetic electrically conducting alloys such as Mu-Metal or the like for optimum performance. For proper operation of the switch, at least the electrical contact 108 must be non-magnetic so that no magnetic forces are developed therewith.
In the presence of a magnetic field, magnetic poles of opposite polarity are induced on the opposing sides of magnetically active electrical contact members 104 and 112. The resulting magnetic attraction overcomes the opposing mechanical spring force so that the electrical contact 104 bends to make electrical contact with electrical contact 112, as shown in FIG. 13. When the magnetic field is removed, the electrical contact returns to its original position electrical contact with electrical contact 108.
In selecting the materials, one desires materials with good electrical conductivity and magnetic properties which allow the activation of the switch. However, these two requirements conflict with each other. Magnetically active materials are not good conductors and good conductors are not magnetically active. Reed switch manufacturers generally optimize these requirements by electroplating the electrical contact area with Rhodium or Ruthenium. However, this combination is particularly sensitive to electrical arcing. As a result, a hermetically sealed glass envelope filled with an inert glass is required to prevent corrosion of the electrical contacts. Even in a hermetic environment, maintaining consistent electrical contact pressure is difficult.
The necessity of the hermetic seal of the glass envelope results in numerous disadvantages. For example, reed switches are highly susceptible to damage. Also, any manipulation or jarring of the lead wires will destroy the seal joint. Further, if the reed switch is dropped, the glass envelope will likely be broken. Moreover, a hermetically sealed environment prevents manufacturing of a magnetic proximity switch having adjustable sensitivity or electrical contact pressure.
Additionally, reed switches suffer from problems in that their size cannot be easily miniaturized. If one attempts to reduce the size of the switch, either the actuation gap range would diminish or the false alarm rate would increase. Furthermore, reed switches suffer from wear and deterioration of the spring mechanisms, and mechanical complexity.
Due to constraints imposed by the inherent structure of reed switches, incorporation of electrical contacts is compromised. Consequently, the life expectancy is extremely sensitive to operating conditions. Also, reed switches are sensitive to magnetic fields, thereby making them susceptible to extraneous fields generated by outside magnetic fields. This characteristic is further exasperated by the introduction of permanent biasing magnets used to polarize the reed switches for increased sensitivity. These problems cause unreliable performance, false alarms, or catastrophic failures, thereby resulting in increased costs and lack of trust in the corresponding security systems.
Because the read switch suffers from the above-noted limitations and disadvantages, an alternative design is needed.